Many current power systems employ DC power supplies having DC/DC power converters located on circuit cards (e.g., load cards) which are directly coupled to voltage buses. The voltage buses generally supply voltages of a magnitude of around 40 V to 60 V. The load cards are usually coupled to the voltage bus and to a separate load.
For system reliability, the load cards should be able to connect to and disconnect from the voltage bus without disturbing the operation of other cards and the bus voltage. Furthermore, if a fault condition occurs in a card, other cards should be electrically isolated from the condition until the fault condition is remedied. Typically, an isolation diode and local bus capacitors are mounted on the cards to accomplish the electrical isolation.
Unfortunately, using local capacitors creates an in-rush current that occurs at the moment when the card is coupled to the voltage bus. The narrow-pulsed and high amplitude in-rush current commonly causes current stresses on the associated electrical components and electromagnetic interference in the system as a whole. Therefore, the current that rushes into the local bus capacitors must be introduced in a controlled manner. The in-rush current is commonly regulated using an RC timing circuit in conjunction with a switching component, such as a metal-oxide semiconductor field-effect transistor (MOSFET) . The turn-on timing of the switch is controlled by the timing circuit. When the card is initially connected, the RC circuit begins charging and will slowly allow the switch to reach its turn-on threshold. As the switch turns on, the in-rush current is allowed, via the switch, to begin charging the capacitors at an acceptable rate.
The in-rush current, however, often cannot reach a satisfactory level due to component tolerances. For instance, the threshold voltage for turning on a MOSFET switch is normally about 2 VDC to about 4 VDC. Capacitance tolerances of the capacitor in the RC timing circuit are approximately .+-.20%. Furthermore, the charging rate of the capacitor is high at low capacitor voltages and low at high capacitor voltages. Therefore, at lower turn-on threshold voltages, the switch begins to conduct very quickly; conversely, the switch begins to conducts more slowly (i.e., less current) at high turn-on threshold voltages. When the switch turns on quickly, a dip in the bus voltage could occur. Such fluctuations in the bus voltage are unacceptable in systems that require high reliability.
When the in-rush current is too large, the switch will suffer a short circuit condition. A fuse is normally employed to protect against short circuit situations in higher reliability systems. When a fuse is connected in series with a semiconductor switching device, such as a MOSFET, it is not always able to detect a short circuit or protect the switch from damage.
Accordingly, what is needed in the art is a protection circuit capable of providing overvoltage, current limit and short circuit protection for a power converter that overcomes the deficiencies in the prior art.